Differences in injection systems (Audi A6 C4)

The MPI injection system has a switchable, two-piece intake manifold, while the MPFI system has a non-switchable, three-piece plastic intake manifold. In addition, the MPFI system no longer requires a mass air flow meter.

Fuel is sucked from the fuel tank by an electric pump and fed through the fuel filter to the dispenser, which is made in the form of a ring fuel line, and then to the valve injectors. The pressure regulator at the end of the dispenser maintains constant fuel pressure in the system.

The valve injectors inject fuel sequentially into the intake manifold before the intake valves. The injection is not simultaneous. This means that the injectors are separated, controlled individually and inject fuel in accordance with the sequence of flashes before the engine intake valves. By selecting the start of injection in relation to the opening moments of the intake valves, the engine's performance and environmental characteristics can be better controlled. In addition, the engine's responsiveness is improved.

Unlike the KE injection system, the MPI system has a mass air flow meter built in instead of a volumetric flow meter. This meter has the following advantages: small dimensions, automatic compensation for the influence of temperature and altitude, no moving parts and therefore low wear.

In the mass flow meter, a filament heated by an electric current is cooled by the flow of intake air passing by. In order to maintain the temperature of the hot filament constant, the heating current varies according to the amount of air flowing through. Based on the change in heating current, the control unit determines the engine load mode and adjusts the amount of fuel injected accordingly. To avoid distortion due to contamination, the filament is heated automatically after each engine shutdown.

The control unit regulates the injection duration and thus the amount of fuel injected in accordance with the signal from the air mass flow meter and the crankshaft speed. With a longer opening of the valve injector, more fuel is injected. Additional sensors and converters ensure that the amount of fuel injected is measured correctly in any driving situation.